Optical components are extensively used in data communication networks. Typically these components are designed to manipulate light having a single mode. Advances in optical technology have provided integrated optical devices that permit more complex optical components and combinations of optical components to be produced on a single optical chip. When the optical chip relies of on weakly guiding waveguides, it is often difficult to provide a curved waveguide whose radius of curvature is sufficiently low to permit a set of optical components on a same to chip to be located in close proximity to each other. Additionally, when the radius of curvature of a curved waveguide is high it often becomes difficult to position the waveguides such that the surface area of substrate is used effectively. Since the substrate is likely to be very costly, it is very beneficial to use a minimal amount of surface area.
While advances in total-internal-reflection (TIR) mirrors allow turning mirrors to be disposed on optical substrates in some applications it is often the case that these mirrors have insertion loss in the order of 1.0 dB. While this may be acceptable in some applications it is not acceptable for others and generally it is beneficial to minimize the insertion loss of optical components.
In U.S. Pat. No. 4,983,006, Hishimoto describes a polarization independent optical waveguide switch. The switch features two curved waveguides that approach each other forming an “X” shape in a top view of the device. This patent clearly demonstrates the usefulness of tightly curved waveguides in optical switching. The waveguides used in this prior art patent are widened near the junction where they are parallel. Hishimoto explains that a higher index contrast is desirable in a curved waveguide in order to enhance confinement of the optical signals as they propagate within the curved waveguide. While the thicker waveguides decrease radiation loss, and hence suggest improved confinement, the radius of curvature used by Hishimoto is still relatively large.
In U.S. Pat. No. 5,511,142, Horie et al. discuss a variety of different ridge waveguide structures intended for use with curved waveguides. The ridge waveguides described by Horie et al. have sections that are not flat. Thus, the ridge is designed to enhance confinement of light propagating within the curved waveguide on the inside of the curve of the waveguide while providing weaker guiding on the outside the waveguide thereby assisting the redirection of the light around the curve. While this prior art is superior to a conventional flat ridge waveguide the enhancement in terms of minimum radius of curvature is modest because the waveguide is still a weakly guiding waveguide. Additionally, it is felt that the processing of such a waveguide is more complex than the processing of a move conventional waveguide device.
In the paper “Air trenches for sharp silica waveguide bends”, IEEE Journal of Lightwave Technology, v. 20, p. 1762 (2002), the authors M. Popovic et al. suggested an adiabatic taper from a low-index contrast to high-index contrast waveguide structure combined with a high-index contrast bend. Along with the obvious advantages of the structures especially for the implementation of the small radius bends, there are several drawbacks. Namely, there is a junction between the different waveguide structures where a mode mismatch loss is likely to be very significant. Additionally, the fabrication of the taper is questionable because it requires very accurate alignment of the masks used at different etching processes.
It would be beneficial to provide integrated waveguide substrates having curved waveguides, the curved waveguides having a relatively low radius of curvature and low optical losses. Further, it would be beneficial to provide this type of waveguide without resorting to unconventional and costly production techniques.